Information recording medium

ABSTRACT

An information recording medium wherein an optically detectable information recording medium is comprised of at least a substrate, a recording layer, and a resin layer. The information recording medium has surface roughness Rσ on the surface of the recording layer, which is in contact with the resin layer, under 5 nm. The information recording medium controls the reproduction signal noise, enables highly densified recording.

This is a continuation of application Ser. No. 09/852,441, filed May 9,2001, now U.S. Pat. No. 6,633,533.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording medium whereina reproducing apparatus uses a relative motion with respect to therecording medium to read information, and particularly to an informationrecording medium wherein optical means are used to carry out recordingand/or reproducing process.

2. Description of the Related Art

Currently there existed various systems which read information widthrelative motion of an information recording medium, and these systemsuse recording medium in shapes of a disk, a card, or a tape. Theinformation recording medium in the form of a disk, especially with anoptically recording and/or reproducing system, is heavily involved inour daily life. For example as a reproduction-only information recordingmedium which uses light with wavelength of 650 nm, there are DVD videoswith pre-recorded visual information, DVD-ROMs with pre-recordedprograms and such, and SACDs and DVD audios with pre-recorded audioinformation. In addition there are DVD-Rs as an addablerecording/reproducing information recording medium using dye, DVD-RAMsand DVD-RWs as a recording/reproducing information recording mediumusing phase change, and ASMO, iD, and GIGAMO as a recording/reproducinginformation recording medium using magneto-optics.

Shortening the wavelength of a laser has been researched over the yearsin order to improve the recording density. The recording density isdetermined by MTF (spatial frequency) of an optical system, and it isexpressed by the equation λ/4NA (λ: reproduction wavelength, NA:numerical apertures of an objective lens). Since a pit length (or a marklength) at the reproduction limit is λ/4NA, reducing λ and increasing NAis necessary in order to increase the recording density.

Recently invented gallium nitride compound semiconductor light emittingelement (Japanese Patent No. 2778405, for example) emits light nearλ=405 nm. It seems to be a very important light-emitting element tosignificantly increase the recording density. The research forcorresponding objective lens for the wavelength near the similar rangeis also progressing, especially with NA over 0.7 under development.

Development of such information recording medium reproducing apparatuswith small λ and NA over 0.7 is also progressing, and combination ofthese technologies gives hope to developing an optical disk withsignificantly increased recording capacity compared with the currentDVDs.

A research to reduce the thickness of a transmission layer to transmitlight, as NA becomes larger, to reduce aberration in case theinformation recording medium tilts is concurrently progressing.

SUMMARY OF THE INVENTION

It is a object of the present invention to provide an improvedinformation recording medium.

In order to achieve the above object, the present invention provides,according to a first aspect thereof, an information recording mediumwherein an optically detectable information recording medium iscomprised of at least a substrate, a recording layer, and a resin layer,and has surface roughness Rσ on the surface of said recording layer,which is in contact with said resin layer, under 5 nm.

According to a second aspect of the present invention is based on thefirst aspect thereof, there provided an information recording mediumwherein said substrate is in contact with said recording layer, andmicro pattern is formed on said recording layer surface.

According to a third aspect of the present invention is based on thefirst aspect thereof, there provided an information recording mediumwherein said recording layer has highly reflective material.

According to a fourth aspect of the present invention is based on thefirst aspect thereof, there provided an information recording mediumwherein said recording layer has dye material.

According to a fifth aspect of the present invention is based on thefirst aspect thereof, there provided an information recording mediumwherein said recording layer has magneto-optical material.

Other object and further feature of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional diagram of an information recordingmedium according to the first embodiment of the invention.

FIG. 2 shows a cross-sectional diagram of an information recordingmedium according to the second embodiment of the invention.

FIG. 3 shows a block diagram of an information recording mediumreproduction apparatus related to the first embodiment of the invention.

FIG. 4 shows a relationship between surface roughness Rσ of a recordinglayer surface of an information recording medium and C/N.

FIG. 5 shows a cross-sectional diagram of an ordinary informationrecording medium according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventor made an actual trial production of the reproducingapparatus with λ approximately 405 nm (gallium nitride compoundsemiconductor light emitting element) and NA over 0.7. In addition atrial production of the information recording medium with high-densityrecording and, at the same time, with reduced thickness of a lighttransmission layer was also made. These two were then combined toattempt an actual reproduction, and it became clear that there weresignificant noises in the reproduction signals. The amount of the noiseis more than anticipated from a reproduction with λ over 650 nm, such asDVDs. Even recording high-density signals on the information recordingmedium degrades reproduction signal S/N and reduces the signal qualitytremendously. It is due to this noise that the recording densitypredicted from DVDs could not be established, and the recording densityhad to be compromised to lower levels.

It requires about 20 to 25 GB of recording capacity if, for example, 2.5hours of high-definition television images were compressed into an MPEGformat for recording. If universally distributed 120 mm disks, which areadopted as CDs and DVDs, were to have such recording capacity,compromising the recording density for the reasons mentioned previouslyis unacceptable.

The inventor investigated and discovered the cause for the noise. FIG. 5is a cross-sectional diagram showing the structure of a typicalinformation recording medium. The information recording medium 10 iscomprised of substrate 1, recording layer 2, and resin layer 4. Theresin layer 4 is thinly constructed (thickness of 0.080 to 0.120 mm, forexample) to correspond to high NA, and the light used for recordingand/or reproducing is emitted through here.

Formation of the recording layer 2 by depositing on thin and flexibleresin layer 4 causes deformation, so the deposit is done on thesubstrate 1 (thickness of 0.3 to 2.0 mm, for example). Therefore thesubstrate surface 1A that is in contact with the recording layer 2becomes the starting point of the deposit and the surface 2A throughpredetermined thickness becomes the terminal point of the deposit.

In other words, the light used for recording and/or reproducing injectsthe recording layer surface 2A after passing through the resin layer 4.Predetermined functions (light absorption for recording or lightreflection for reproduction) take place on the entire recording layer 2,then the light returns to a light pickup (not shown) after passingthrough the resin layer 4. The recording surface layer 2A is rough andwith the selection of such structure, the surface roughness of therecording layer surface 2A superposition as a noise directly to thesignals. Hence, the main reason for the noise is found on the recordinglayer surface 2A. The roughness has become so enhanced that it is at thelevel where it can not be disregarded following the reduction of thespot size created by λ of reproduction pickup and NA. That is, theroughness relative to the reproduction spot size becomes toosignificant. This roughness was the reason for the recording density notimproving as much as predicted. The improvement of the recording layer 2of the information recording medium is sought, and setting suitabledegree of the surface roughness of the recording layer surface 2A andits design guidelines are desired.

First Embodiment

The invention is explained in detail using the accompanying drawings.The premises for this invention, the information recording medium 100,are an optical system with laser wavelength λ=370 to 430 nm (near 405nm, for example) and numerical apertures in the objective lens NAbetween 0.7 to 0.9. Especially the noise element of the laser itselfmust be in RIN between −115 to −135 dB/Hz range. RIN represents awell-known laser noise measurement, Relative Intensity Noise.

FIG. 1 is a cross-sectional diagram of the most basic structure of theinvention, the information recording medium 100. The invention, theinformation recording medium 100, is comprised at least of the substrate1, the recording layer 20, and the resin layer 4. The substrate 1 iscomposed of either resin composites, ceramics, or metals. Examples ofthe resin composite which can be used preferably include thermoplastic,thermosetting, or radiation curable resin (including ultra-violetcurable resin and visible radiation curable resin) such aspolycarbonate, polymethyl methacrylate, polystyrene,polycarbonate-polystyrene copolymer, polyvinyl chloride, alicyclicpolyolefin, and polymethyl pentene. It could be a resin composite withmetallic powder and/or ceramic powder combined. Examples of the ceramicsinclude soda lime glass, silicate glass, borosilicate glass, or quartzglass. The thickness of the substrate 1 should be between 0.3 to 3 mm,preferably between 0.6 to 2 mm.

If the information medium is a reproduction-only information recordingmedium, then the recording layer 20 has highly reflective recordingmaterial. If it is a reproducing/recording type information recordingmedium, then according to its recording or reproducing principle, dye,phase change, or magneto-optical material is used. The thicknesspreferably used is between 20 to 200 nm.

Highly reflective recording material with reflectivity over 70%,preferably over 80%, at the reproducing wavelength λ can be used. Amongthe choices are metals such as aluminum, silver, silicon, titanium,chromium, nickel, tantalum, molybdenum, iron, gold, copper and theiralloys (The alloys include oxides, nitrides, carbides, sulfides, andfluorides). Aluminum or Silver alloys are especially preferable for theuse.

Specific examples of the dye material for a dye recording includecyanine dye, phthalocyanine dye, naphtalocyanine dye, azo dye,naphtoquinonedye, fulgide dye, polymethyne dye, and acridine dye.

Specific examples of the phase change recording material include alloysof indium, antimony, tellurium, selenium, germanium, bismuth, vanadium,gallium, platinum, gold, silver, copper, tin, and arsenic (The alloysinclude oxides, nitride, carbides, sulfides, and fluorides). It isespecially preferable to use GeSbTe, AgInSbTe, or CuAlTeSb. Using thelaminating film of indium alloy and tellurium alloy for the recordinglayer 20 is acceptable as well.

Specific examples for the magneto-optical recording material includealloys of terbium, cobalt, iron, gadolinium, chromium, neodymium,dyspromium, bismuth, palladium, samarium, holmium, praseodymium,manganese, titanium, erbium, ytterbium, lutetium, and tin (The alloysinclude oxides, nitride, carbides, sulfides, and fluorides). It isespecially preferable to use alloys with transitional metals andrare-earth elements, such as TbFeCo, GdFeCo, or DyFeCo. In additionusing alternating laminating films of cobalt and platinum for therecording layer 20 is acceptable as well.

It is also acceptable to laminate a combination of support films, suchas silicon, tantalum, zinc, magnesium, calcium, aluminum, chromium, orzirconium alloys (including oxides, nitrides, and carbides) and highlyreflective films (aluminum, gold, silver, etc.) in order to improve thereproduction output, re-writing limits, recording stability, etc.

The main point of this invention is to set the surface roughness of thesurface 20A of the recording layer 20, in contact with the resin layer4, under the predetermined value, specifically Rσ under 5 nm andpreferably under 2 nm. Rσ is known as “root mean square roughness”,defined by Australian standard AS1965-1977. Among several definitions toexpress surface roughness, this expression has the compatibility withthe reproduction principle of an information recording medium thatreproduces optical phase differences, hence it is the optimum expressionto equalize the surface 20A as a noise. Prior to accepting thisstandard, the inventor confirmed the strong correlation between this Rσand the noise output of the information recording medium. More detailsregarding this definition is explained on page 18 of JIS-B0601-1982.

For the resin layer 4, thermosetting resins, several radiation curableresins (including ultra-violet curable resins and visible radiationcurable resins), electronic beam curable resins, humidity curableresins, multi-liquid mixture curable resins, and thermoplastic resinscan be used. The thickness range of the resin layer 4, consideringpreviously discussed optical parameters, lies between 0.080 to 0.120 mm.The maximum variations on the surface of the information recordingmedium is ±0.003 mm, preferably ±0.002 mm, and under ±0.001 would evenbe better. The optical characteristic of the resin layer is choosing amaterial wherein the transmissivity is 60% or more, and preferably 80%or more, with the reproducing light and/or the recording light atwavelength λ. In addition, the double pass birefringence, with thereproducing light or the recording light at wavelength λ, is under ±20nm, and preferably under ±15 nm. The resin layer 4 forms with respect tothe recording layer 20 by fusion or adhesion. For this purpose, it isacceptable for the layer 4 to be of multiple layers of laminations.

The invention of the information recording medium 10 was explained aboveusing FIG. 1. The invention is an information recording medium comprisedof at least the substrate 1, the recording layer 20 with the surface 20Ain contact with the resin layer 4, and the resin layer 4. Itscharacteristic is the surface roughness of the surface 20A under apredetermined value, or Rσ is under 5 nm. Such roughness is usually notconsidered upon depositing the recording layer 20, but this invention isestablished by selecting materials and deposit conditions to achievesuch value. For example with the case of a reproduction-only informationrecording medium, aluminum is ordinarily used. An aluminum alloy with anaddition of small amounts of elements prevents enlargement of particlesand keeps the surface roughness within the predetermined value.

Second Embodiment

The micro pattern P may be formed on the surface 20A, in contact withthe recording layer 20 of the substrate 1, as an application of thisinformation recording medium 10 as shown in FIG. 2. The micro pattern Pis a track pattern formed either in straight lines, concentric circles,or spirals, continuous grooves as seen macroscopically, intermittentpits as seen microscopically, or combinations of the above. The size isdetermined so that, for example, the groove widths and/or the pit widthslies between 0.05 to 1.0 μm.

Other Embodiment

Though not shown in figures it is acceptable to prepare two informationrecording medium 100, either with structures of FIG. 1 or FIG. 2,mutually face the substrates 1, and stick them together. It is alsoacceptable to add another set of layers of the recording layer 20 andthe resin layer 4 on top of the resin layer 4 of the informationrecording medium 100 with structures of FIG. 1 or FIG. 2. This wouldapproximately double the capacity of the information recording medium100.

[Information Recording Medium Reproducing Apparatus]

The detailed structure of the information recording medium reproducingapparatus of this invention, and the reproduction operation is explainednext. FIG. 3 shows the typically illustrated information recordingmedium reproducing apparatus. In other words, it is the informationrecording medium reproducing apparatus 200 loaded with the informationrecording medium 100 structurally mentioned above, and it has thereproducing means to reproduce the information recorded. The reproducingmeans comprises of at least the light pickup 50, the motor 51, the servosection 52, the turntable 53, the demodulator 54, the interface (I/F)55, and the controller 60.

The light pickup 50 comprises of at least a light emitted at singlewavelength λ between 370 to 430 nm, a laser with a noise component RINbetween −115 to −135 dB/Hz, and numerical apertures NA between 0.7 to0.9. They help form the convergence reproduction light. The light pickup50 also has a built-in photo detector to be able to gather thereflective light from the information recording medium 100.

Basic structures and reproducing operations of the information recordingmedium reproducing apparatus 200 is explained using FIG. 3. The lightpickup 50 condenses the convergence light beam 70 to the informationrecording surface of the information recording medium 100, detects therecording signals by gathering the reflecting light, and then sends thesignals to the demodulator 54. Focus-error signals and tracking-errorsignals from the reflecting light are sent to the servo section 52. Theservo section 52 then creates focus-servo signals and tracking-servosignals, controlled by the controller 60, and sends them to the lightpickup 50. The servo section 52 creates rotating-servo signals as well,which is sent to the motor 51. The motor 51 is connected to theturntable 53 and the turntable 53 and the information recording medium100 are connected by fitting in the center hole H. The motor 51, throughthe turntable 53, holds the information recording medium 100 andprovides relative motion necessary for reproduction.

The demodulator 54 demodulates the recording signals, corrects theerrors when necessary, then sends the acquired data stream to theinterface (I/F) 55. Then it sends the signals outside based on theinstructions from the controller 60. It is acceptable to connect thesignal output to an external output terminal not shown in figures, ordirectly connect to a displaying apparatus, a speaker apparatus, or aprinting apparatus.

The information recording medium reproducing apparatus 200 is loadedwith the information recording medium 100, and they are designed toconform with the reproduction light produced by a single-wavelengthlight emitted at λ between 370 to 430 nm, a laser with noise componentRIN between −115 to −135 dB/Hz, and numerical apertures NA between 0.7to 0.9. The information recording medium 100 is preferably reproducedunder these conditions. The turntable 53 and the information recordingmedium 100 are connected by fitting in the center hole H, but thisconnection can be fixed or detachable, which connects/detaches freely.

Details of the invention are further explained based on thecorresponding trial products of the information recording medium 100.

[Sample 1 of Trial Product]

The disk-shaped information recording medium 100, 120 mm in diameterwith structures of FIG. 2, is prepared as a reproduction-onlyinformation recording medium. Thermoplastic resin polycarbonate, 1.1 mmin thickness, is used as the substrate 1. Single frequency signal (pit)with pit length of 220 nm is recorded spirally as the micro pattern P onthe surface. The recording layer 20 is formed by deposit usingsputtering method on the micro pattern P. The surface roughness Rσ ofthe surface 20A of the recording layer 20 is measured using an atomicforce microscope (roughness corresponding to the micro pattern P isdisregarded, however). Thermoplastic resin polycarbonate sheet andultraviolet-curable resin are prepared for the resin layer 4. Therecording layer surface 20A and the polycarbonate are piled through theultraviolet-curable resin for ultraviolet curing to complete theinformation recording medium 100.

By repeating the process with different materials, various metals withreflectivity over 80% at λ, for the recording layer 20, several samplesof the information recording medium 100 were prepared.

The information recording medium reproducing apparatus 200 with thelight pickup 50, comprised of a laser illuminating at λ of 405 nm, anoise component RIN at −115 dB/Hz, and numerical apertures NA at 0.7 isprepared. The state of the information recording medium 100 loaded tothe reproduction apparatus 200 is shown in FIG. 3. Reproduction of themicro pattern P signals is tested by spinning the information recordingmedium 100. The reproduction signals are then measured for C/N(carrier-to-noise ratio) using a spectral analyzer with resolvingbandwidth set at 30 kHz.

FIG. 4 shows the relationship between the surface roughness Rσ, of therecording layer surface 20A, and C/N for each sample. There is a strongcorrelation between Rσ and C/N, as shown in the figure, which can beexpressed in a binary power series. Areas of small Rσ provide sufficientC/N but at certain values, or at Rσ over 5 nm, C/N tends to digressdrastically. As a matter of a fact C/N can not extract the signals at 40dB and the value over 45 dB becomes desirable, so even from thisstandpoint Rσ should at least be less than 5 nm.

Considering these tendencies, partially revised information recordingmedium 100 is shown in the embodiment that follows.

[Sample 2 of Trial Product]

D8-15 modulation (noted in Japanese Patent Application No. 2000-020171)recorded spirally is used as the micro pattern P of the informationrecording medium 100. The modulation signals are integral multiplelength signals from 3T to 11T, and the shortest pit 3T is set at 220 nmin length. The interval of the spiral track is 341 nm. The physicaldimension for a disk with 120 mm in diameter would provide 20 GB ofrecording capacity. An aluminum alloy A6061 is used as the recordinglayer 20, formed 45 nm in layer thickness by sputtering in argon gas.The surface roughness Rσ of the surface 20A was 0.95 nm at this time(the roughness of the micro pattern P is disregarded, however).

By loading this information recording medium 100 to the informationrecording medium reproduction apparatus 200 to attempt reproduction,signal outputs were 79% 11T modulation and 23% 3T modulation. Thesesignals were decoded using an error selecting equalizer of JapanesePatent Application No. 11-291634/1999 (equal to U.S. patent applicationSer. No. 09/468,130 filed on Dec. 21, 1999 and European PatentApplication No. 99125733.8 filed on Dec. 23, 1999) and an adaptiveViterbi decoder of Japanese Patent Application No. 11-155673/1999 (equalto U.S. patent application Ser. No. 09/438,297 filed on Nov. 12, 1999and European Patent Application No. 99309596.7 filed on Nov. 30, 1999).The signals output were consistently at low error rate of 5×10⁻⁵.

The measurement of an angle (a tilt margin) for the system breakdown ismeasured by tilting the information recording medium 100, and there werelatitudes of ±0.6 degrees radial adjusting and ±0.5 degrees tangentialadjusting. These results would provide sufficient degree of margin evenafter considering sporadic productions of the information recordingmedium 100 and/or the turntable 53 and curvature generations in userenvironment.

[Comparison Sample]

The structural elements are identical to the Sample 2 except for 90 nmin layer thickness of pure aluminum as the recording layer 20. Thesurface roughness of the surface 20A was 5.5 nm (roughness correspondingto the micro pattern P is disregarded, however).

The reproducing signal had high error rate of 3.5×10⁻⁴ and showedtendencies to intermittently halt the decoding process. It is thereforeactually at the level of a non-working system. Slight tilt of theinformation recording medium 100 significantly aggravated the error rateas well.

Embodiments of the invention have been discussed. Based on the conceptof the invention, alternative forms of the application are possible. Thestructural elements mentioned throughout may be mutually exchangedwithin the boundaries not impeding the invention contents. For example,previously described information recording medium 100 was in adisk-form, but it could be in the form of a card or a tape. Theinformation recording medium 100 may be attached inside a cartridge. Thesize of the disk is not limited to 120 mm in diameter, but rather may be40 to 300 mm in diameter; or 51, 65, 80, 88, 130, 200, 300 mm indiameter, for example.

The signals recorded on the micro pattern P are not limited to the D8-15modulation. Several modulation signals of so-called (d,k) codes may beused instead. Both fixed-length codes and variable-length codes areacceptable. Both mark-edge recordings and mark-position recordings areeffective. A mark-edge recordings recording and a type of fixed-lengthcodes, (2.10) RLL modulation (8/16 modulation or 8/17 modulation, forexample) or a mark-edge recording and a type of variable-length codes,(2.7) modulation or (1.7) modulation, are especially possible for theuse.

According to the aspect of the present invention, there provided aninformation recording medium, limiting the surface roughness Rσ, of arecording layer surface of the information recording medium, under 5 nmcontrols the reproduction signal noise, enables recording of over 20 to25 GB of information onto an ordinary sized information recordingmedium, and materializes highly densified recording medium.

1. A reproducing apparatus for an optically detectable informationrecording medium comprising at least a substrate, a recording layer, anda resin layer, wherein the surface of said recording layer in contactwith said resin layer has a Root Mean Square roughness Rσ of less than 5nm, said reproducing apparatus at least comprising: a turntable forholding said information recording medium; a motor connected to saidturntable for making said turntable move relatively; a light pickup forconverging light on said information recording medium and for receivingreflected light from said information recording medium; a demodulatorfor demodulating a signal from said light pickup and for transmitting ademodulated signal to an interface; a servo controller for generating afocus servo signal to drive said light pickup and a tracking servosignal; an interface for transmitting said demodulated signal from saiddemodulator externally; and a controller for controlling said motor,servo controller, and interface.
 2. The reproducing apparatus inaccordance with claim 1, wherein said light pickup is at least composedof an objective lens having a numerical aperture of 0.7 to 0.9 and aphoto detector.
 3. An optically detectable information recording mediumat least comprising: a substrate; a recording layer; and a resin layer,wherein the surface of said recording layer in contact with said resinlayer has a Root Mean Square roughness Rσ of less than 5 nm, and saidsubstrate and said recording layer is in contact with each other, andthe surface of said substrate is formed with micro patterns, and saidmicro patterns further comprise a pit.
 4. The information recordingmedium in accordance with claim 3, wherein width of said pit is 0.05 μmto 1.0 μm.
 5. The information recording medium in accordance with claim3, wherein said pit is modulated by a modulation signal through the RLLmodulation method.
 6. An optically detectable information recordingmedium at least comprising: a substrate; a recording layer; and a resinlayer, wherein the surface of said recording layer in contact with saidresin layer has a Root Mean Square roughness Rσ of less than 5 nm, andsaid substrate and said recording layer is in contact with each other,and the surface of said substrate is formed with micro patterns, andsaid micro patterns further comprise a groove and a land.
 7. Theinformation recording medium in accordance with claim 6, wherein widthof said groove is 0.05 μm to 1.0 μm.